1. Field of the Invention
The present invention relates to bipolar transistor manufacturing methods and, in particular, to methods for controlling an N-type dopant concentration into the subsequent P-type base layer grown epitaxially right after the N-layer growth.
2. Description of the Related Art
Silicon (Si) and Silicon/Silicon-Germanium (Si/SiGe) epitaxial-base layers are commonly employed in bipolar junction transistors (BJTs) and heterojunction bipolar transistors (HBT""s), respectively. When these transistors are to be used in high frequency performance wireless, radio-frequency (RF) and communication devices, their small-signal unity gain frequency (fT) and maximum oscillation frequency (fmax) performance can be in excess of 40 GHz. See, for example, D. L. Harame, et al., Si/SiGe Epitaxial-Base Transistors-Part I: Materials, Physics, and Circuits, in IEEE Transactions on Electron Devices, Vol. 42, No. 3, pp. 455-486 (March, 1995) and D. L. Harame, et al., Si/SiGe Epitaxial-Base Transistors-Part II: Process Integration and Analog Applications, in IEEE Transactions on Electron Devices, Vol. 42, No. 3, pp. 469-482 (1995), both of which are hereby incorporated by reference, for a further discussion of the science and technology of epitaxial-base BJT""s and HBT""s.
Attaining high frequency performance in BJT""s and HBT""s requires optimization and control of the dopant concentration depth profile in both the base region and the underlying collector region. For example, in the base/collector structure of an NPN BJT or HBT, it is desirable to control the n-type phosphorus (or arsenic) dopant concentration depth profile in regions immediately underlying a p-type base layer.
Unfortunately, the use of dopant ion implantation and thermal drive-in processes for the doping of bipolar transistor epitaxial layers can result in undesirable implant channeling and unacceptably wide and non-abrupt dopant concentration depth profiles in the base region. Furthermore, when phosphorus in-situ doped epitaxial layers are used as a portion of the collector region in high-frequency epitaxial base layer BJT""s or HBT""s, phosphorus is observed to unintentionally accumulate in overlying p-type layers (i.e. boron-doped silicon and boron-doped SiGe layers), thereby increasing the n-type dopant concentration in the p-type layers to an undesirable level.
The abruptness of arsenic and phosphorus dopant concentration during the onset of in situ doped silicon epitaxy has been reported in the literature. See T. I. Kamins and D. Lefforge, xe2x80x9cControl of n-Type Dopant Transitions in Low-Temperature Silicon Epitaxyxe2x80x9d J. Electrochemical Soc., Vol 144, No. 2, pp. 674-678 (Feb. 1997), which is hereby fully incorporated by reference. Methods for controlling the n-type dopant concentration depth profile in undoped or p-type doped epitaxial layers formed subsequent to an n-type in-situ doped epitaxial layer are, however, not known in the art.
Still needed is a method for the formation of NPN bipolar transistor epitaxial layers, including undoped or p-type doped epitaxial layers formed subsequent to an n-type in-situ doped epitaxial layer, with a controlled n-type dopant concentration depth profile.
FIG. 1 is a cross-sectional representation of a portion of an NPN BJT 100 that includes a semiconductor substrate 102, an n-type collector precursor region 104 formed on the surface of the silicon substrate, and silicon dioxide (SiO2) isolation layers 106 formed (for example, by a LOCal Oxidation of Silicon [LOCOS] technique) on the surface of the semiconductor substrate 102. Overlying the n-type collector precursor region 104 are an n-type in-situ doped epitaxial layer 108 (which will become part of a final NPN BJT collector region) and a p-type in-situ doped epitaxial base layer 110. Polysilicon layers 112 and 114, formed during deposition of the n-type in-situ doped epitaxial layer 108 and the P-type in-situ doped epitaxial base layer 110, respectively, over the SiO2 isolation layers. An idealized dopant concentration depth profile for n-type in-situ doped epitaxial layer 108 and P-type in-situ doped epitaxial base layer 110 is illustrated in FIG. 2.
FIG. 3 is a cross-sectional representation of a portion of an NPN HBT 200 that includes a semiconductor substrate 202, an n-type collector precursor region 204 formed on the surface of the semiconductor substrate 202, and silicon dioxide (SiO2) isolation layers 206 formed on the surface of the semiconductor substrate 202. Overlying the n-type collector precursor region 204 are an n-type in-situ doped epitaxial layer 208 (which will become part of the NPN HBT""s collector region) and an epitaxially grown SiGe layer (doped or undoped) with Boron doped cap layer 210. Polysilicon layers 212 and 214, formed during the epitaxial deposition of layers 208 and 210, respectively, over the SiO2 isolation layers. An idealized dopant concentration depth profile for n-type in-situ doped epitaxial layer 208 and p-type in-situ doped epitaxial base layer 210 is illustrated in FIG. 4. Structure 200 is similar to structure 100, however, it further includes a graded Si1xe2x88x92xGex region starting at the end of the layer 208 (N-type epi).
FIG. 5 is a Secondary Ion Mass Spectroscopy (SIMS) dopant (boron and phosphorus) concentration depth profile of an actual structure with a n-type (i.e. phosphorous) in-situ doped epitaxial layer and a p-type (i.e. boron) in-situ doped epitaxial base layer. The arrow at a depth of approximately 1580 angstroms indicates the location where the formation of the n-type (i.e. phosphorous) in-situ doped epitaxial layer began. The arrow at a depth of approximately 575 angstroms indicates the point where the phosphorus dopant source was turned off and the formation of the p-type (i.e. boron) in-situ doped epitaxial base layer began. These actual dopant concentration depth profiles in FIG. 5 correspond to the idealized dopant concentration depth profiles of FIG. 2. As shown in FIG. 5, the phosphorous dopant concentration depth profile does not decrease to zero (as in the idealized dopant concentration depth profile of FIG. 2) even though the phosphorus dopant source was turned off at the start of the formation of the p-type in-situ doped epitaxial base layer. Instead, it tends to increase due to n-type dopant (i.e. phosphorus) accumulation in the overlying p-type layer. Similar n-type dopant accumulation behavior has been observed when a graded Si1xe2x88x92xGex region is present at the epitaxial base layer. As is explained immediately below, this uncontrolled accumulation of n-type dopants in epitaxial layers formed subsequent to (and, therefore, overlying) a P-type in-situ doped epitaxial layer is undesirable for high performance bipolar transistor applications.
It is desirable, with respect to increasing the frequency performance of bipolar transistors, to minimize the collector-base junction capacitance (Ccb) by reducing the dopant concentration at the collector. This is desirable since minimizing Ccb increases the maximum oscillation frequency (fmax). Also to increase Ft of transistors, the base transit time can greatly be reduced by increasing the base doping. High base doping also increases Ccb cap and lowers the gain of the transistor. To optimize this situation, it is desirable to have a low collector doping and less compensation of the base P-type dopant by the N-type collector dopant. To minimize Ccb in NPN bipolar transistors where epitaxial layers (i.e. either a p-type in-situ doped epitaxial base layer in the case of an BJT or an undoped Si1xe2x88x92xGex epitaxial layer in the case of a HBT) are deposited subsequent to formation of n-type in-situ doped epitaxial layer that will become part of the transistor""s collector region, the n-type dopant concentration depth profile in these subsequently formed epitaxial layers should be less than, or equal to, that in the n-type in-situ doped epitaxial layer. That is. Ccb in NPN bipolar transistors is minimized if the n-type dopant concentration depth profile in the underlying n-type in-situ doped epitaxial layer (i.e. a precursor to the transistor""s collector region) exceeds the n-type dopant concentration depth profile in the subsequently deposited p-type in-situ doped epitaxial base layer in a BJT (or undoped Si1xe2x88x92xGex epitaxial layer in a HBT). For example, if the level of phosphorous dopant concentration in an n-type in-situ doped epitaxial layer is 2e17 atoms/cm3, then the phosphorous dopant concentration level in subsequently formed epitaxial layers (such as a p-type in-situ doped epitaxial base layer or an undoped Si1xe2x88x92xGex epitaxial layer) due to the accumulation of such dopant therein should be no greater than 2e17 atoms/cm3. The ideal situation would be where the n-type dopant concentration at the collector-base junction inside the base region becomes zero (as in FIGS. 2 and 4). However, as shown by the actual data in FIG. 5, phosphorus (an n-type dopant) can accumulate in subsequently formed epitaxial layers to concentration levels that are higher than the concentration of phosphorus dopant remaining in the n-type in-situ doped epitaxial layer itself, even though the phosphorus dopant source was turned off (i.e. there was no intentional doping with n-type dopants) when the subsequent formation of the epitaxial layers began.
It is theorized, without being bound, that the accumulation of n-type dopants in epitaxial layers deposited subsequent to the formation of an n-type in-situ doped epitaxial layer is due to n-type dopant segregation at the interface during deposition of the subsequent layers. This segregation of phosphorus is not due to out-diffusion resulted from growth temperature (the growth temperature being very low). Also, this is not a result of memory effect in the reactor (even 13 minutes purge between the growth did not reduce the phosphorus pile-up in the subsequent P-type layer). It is indeed a leaching/segregation effect associated with advancing epi front. Independent of the cause of this unwanted accumulation, however, processes according to the present invention provide for the formation of NPN bipolar transistor epitaxial layers, including undoped and p-type doped epitaxial layers formed subsequent to an n-type in-situ doped epitaxial layer, wherein the n-type dopant concentration depth profile in the p-type doped epitaxial layers is controlled and minimized. Up to an order of magnitude or greater reduction of the accumulated n-type dopant concentration depth profile levels in the p-type doped epitaxial layers, in comparison to conventional methods, can be obtained using processes according to the present invention.
Processes in accordance with the present invention provide for the formation of epitaxial layers with controlled n-type dopant concentration depth profiles for use in NPN bipolar transistors (both BJTs and HBTs). The processes include first providing a semiconductor substrate (e.(g. a [100]-oriented silicon wafer substrate) with an n-type collector precursor region formed on its surface. Next, an n-type (e.g. phosphorous or arsenic) in-situ doped epitaxial layer of a first thickness t1 is formed on the n-type collector precursor region, followed by the formation of an undoped epitaxial layer of a second thickness t2 on the n-type in-situ doped epitaxial layer, as well as the formation of a p-type (e.g. boron) in-situ doped epitaxial base layer over the undoped epitaxial layer. Through out the specification, the term xe2x80x9cundoped epitaxial layerxe2x80x9d refers to epitaxial layers that are not intentionally doped (i.e. dopant sources are turned off during the formation of the subjected epitaxial layers), although the layers may contain accumulated dopants that are segregated from the underlying layers (as explained with reference to FIG. 5 above). The accumulated n-type dopant concentration in a p-type in-situ doped epitaxial base layer formed is controlled by the adjusting the thickness ratio of the latter two layers (i.e. t2 to t1).
Processes according to the present invention can also be applicable for SiGe epi process for heterojunction bipolar transistor. Just splitting the N-layer growth into two regions; one with in-situ phosphorus doped (t1) layer and subsequent undoped (t2) layer. By controlling the ratio of t2 to t1, the pile-up N-dopant into the subsequent SiGe and SiGe with boron doped layers can be controlled.